Transitioning to and from a sleep state of a processor

ABSTRACT

A method for reducing power utilized by a processor including determining that a processor is transitioning from a computer mode to a mode in which system clock to the processor is disabled, and reducing core voltage to the processor to a value sufficient to maintain state during the mode in which system clock is disabled.

RELATED APPLICATION

This application is a Divisional Application of co-pending, commonlyowned U.S. patent application Ser. No. 09/694,433, attorney docketTRAN-P059, filed Oct. 23, 2000 now U.S. Pat. No. 7,260,731, entitled“SAVING POWER WHEN IN OR TRANSITIONING TO A STATIC MODE OF A PROCESSOR”to Read et al., which is hereby incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to computer systems and, more particularly, toapparatus and methods for reducing power use by a computer system duringintervals in which processing is stopped.

2. History of the Prior Art

As computer processors have increased in ability, the number oftransistors utilized has increased almost exponentially. This increasein circuit elements has drastically increased the power requirements ofsuch processors. As the need of power increases, the temperature atwhich a computer operates increases and the battery life of portablecomputers decreases. The loss of battery life with modern portablecomputers greatly reduces the time during which the computer canfunction as a portable device. In fact, the power usage has become sogreat that even with significant reduction in the process size utilized,a plethora of techniques have been implemented to reduce power usage tomaintain the efficacy of portable computers.

One of these techniques monitors the use of the various devices withinthe computer and disables those devices that have not been utilized forsome period. Because the processor utilizes a significant amount of thepower (e.g., 50%) used by a portable computer, this technique isutilized to disable the processor itself when its processingrequirements are unused for some interval. In the typical case,disabling the processor is accomplished by terminating the system clocksfurnished to the processor. When processor clocks have been disabled,controlling circuitry (typically a portion of the “Southbridge”circuitry of an X86-processor-based computer) remains enabled to detectinterrupts requiring processor operation. The receipt of such aninterrupt causes the controlling circuitry to once again enable clocksto the processor so that the processor may take whatever steps arenecessary to handle the basis of the interrupt.

The technique of disabling the processor reduces significantly thedissipation of power caused by the operation of the processor even at alow frequency. In fact, the technique works quite well; and it isestimated that with many portable computers the processor is placed inthe state in which system clocks are disabled during approximatelyninety percent of the operation of the computer. However, use of thistechnique emphasizes another aspect of power loss using advancedprocessors. When system clocks for a processor are disabled, theprocessor must remain in a state (sometimes called “deep sleep”) inwhich it is capable of rapidly responding to interrupts. Such a staterequires the application of core voltage to the various circuits. Theapplication of this voltage generates a power dissipation referred to inthis specification as “static power” usage because the processor is inits static state in which clocks are disabled. To date there has beenlittle attention paid to this static power usage. However, the usage isvery significant when a processor functions in the deep sleep mode asmuch as ninety percent of the time. As process technologies continue toshrink in dimension and lower operating voltages, this static powerincreases due to lower threshold voltages and thinner gate oxides.

It is desirable to furnish apparatus and methods for reducing the poweruse of a processor in the state in which its clocks are disabled.

SUMMARY OF THE INVENTION

The present invention is realized by a method for reducing powerutilized by a processor including the steps of determining that aprocessor is transitioning from a computing mode to a mode in whichsystem clock to the processor is disabled, and reducing core voltage tothe processor to a value sufficient to maintain state during the mode inwhich system clock is disabled.

These and other features of the invention will be better understood byreference to the detailed description which follows taken together withthe drawings in which like elements are referred to by like designationsthroughout the several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating current-voltage characteristics of CMOStransistor devices utilized in microprocessors.

FIG. 2 is another diagram illustrating current-voltage characteristicsof CMOS transistor devices utilized in microprocessors.

FIG. 3 is a circuit diagram illustrating a first circuit designed inaccordance with the present invention for reducing static power usage.

FIG. 4 is a circuit diagram illustrating a second circuit designed inaccordance with the present invention for reducing static power usage.

FIG. 5 is another circuit diagram illustrating a circuit designed inaccordance with the present invention for reducing static power usage.

DETAILED DESCRIPTION

FIG. 1 is a first diagram displaying a number of curves illustrating thecurrent-voltage characteristics of CMOS transistor devices utilized inthe circuits of a microprocessor. This first diagram utilizes a linearscale for both current and voltage. As may be seen, each of the curvesillustrates that the drain-to-source current of a transistor isessentially nonexistent until the voltage at the gate terminal of thetransistor is raised to a threshold voltage. Once the threshold voltageof the transistor is reached, drain-to-source current increases eitherlinearly or quadratically depending on whether the transistor is in thelinear region or saturation region of operation.

Although the diagram of FIG. 1 appears to illustrate that currentflowing below the threshold value of the gate voltage is insignificant,this is not the case in some situations. FIG. 2 illustrates currentversus voltage curves of the typical transistor device below thethreshold voltage with the voltage being plotted on a log scale. As maybe seen, current in fact flows below the threshold voltage. If atransistor functions in the state below the threshold voltage for ninetypercent of computer processor operation, then this current has asignificant affect on power usage by the processor.

Since a processor is not capable of computing in the mode in which itsclocks are disabled, it would at first glance appear that the solutionwould be to terminate the application of voltage to the processor.However, as suggested above, it is necessary that the processor bemaintained in a condition in which it can respond rapidly to interruptsprovided by the circuitry that controls application of the systemclocks. To do this, the processor must maintain state sufficient toimmediately return to an operating condition. Thus, prior art processorshave been provided sufficient voltage to maintain such state and to keeptheir transistors ready to immediately respond to interrupts. Ingeneral, this has been accomplished by maintaining the processor corevoltage at the same level as the operating voltage. With most prior artprocessors, the core voltage used by a processor is selected by use ofmotherboard switches or setup software at a level sufficient to providethe highest frequency operations specified for the particular processor.For example, many processors provide 1.8 volts as a core voltage. On theother hand, the voltage required to maintain state in a deep sleep modemay be significantly less, e.g., one volt or less. Since such processorsfunction at the same voltage whether in a computing or a deep sleepmode, a significant amount of unnecessary power may be expended. In onetypical state of the art X86 processor, the power usage averagesapproximately one-half watt in the deep sleep state because of theleakage illustrated by the diagram of FIG. 2.

The present invention reduces the voltage applied to the processorsignificantly below the lowest voltage normally furnished as a corevoltage for the processor during the mode in which system clocks aredisabled thereby reducing the power utilized by the processor in thedeep sleep state.

FIG. 3 is a circuit diagram illustrating a first embodiment of theinvention. In the circuit 10 illustrated, a switching voltage regulator11 receives an input signal at a terminal 12 which determines its outputvoltage value. Most modern processors utilize a voltage regulator whichis capable of furnishing a range of core voltages for operatingtransistors; a typical regulator may furnish a range of voltages between2 and 0.925 volts from which a particular core voltage may be selectedfor operation. Typically, a binary signal is provided a the terminal 12which selects the particular output voltage level to be furnished by theregulator 11; in such a case, a number of individual pins may beutilized as the terminal 12.

Recently, a new power saving technique has been utilized whichdynamically adjusts both the voltage and operating frequency to a levelsufficient to maintain computing operations being conducted by aprocessor. The technique which offers significant power savings isdescribed in detail in U.S. patent application Ser. No. 09/484,516,filed Jan. 18, 2000, entitled Adaptive Power Control, assigned to theassignee of the present invention. A processor which utilizes thistechnique monitors operations within the processor to determine thefrequency level at which the processor should operate. Depending on theparticular operations being carried out by the processor, the valuefurnished at the terminal 12 of a regulator functioning in such a systemwill cause the regulator to produce an output voltage at some levelbetween the high and low values necessary for the particular processorto carry out computing functions.

In the circuit of FIG. 3, input to the terminal 12 is furnished via acircuit 13 such as a multiplexor that is capable of providing one ormore input values. In the embodiment illustrated, a value is provided ata first input 14 to the circuit 13 by the processor (or other circuitry)which determines the operating condition of the processor in itscomputing range; and a second value is provided at a second input 15which is selected especially for the deep sleep condition. Either ofthese input values may be selected by a control signal provided at acontrol terminal 16 of the circuit 13. In one embodiment, a systemcontrol signal normally utilized to signal entry into the deep sleepcondition (a stop clock signal) is used as the control signal to befurnished at the control terminal 16. This control signal selects theinput value furnished at the input 15 which is especially chosen tocause a typical prior art regulator 11 to produce a voltage output foroperating the processor in the deep sleep mode. In one embodiment of theinvention, the value furnished for deep sleep mode is chosen to causethe regulator 11 to produce the lowest voltage possible in its range ofoutput voltages. In one exemplary processor that utilizes the techniquedescribed in the above-mentioned patent application, the processor isspecified as capable of conducting computing operations in a corevoltage range from a low voltage of 1.2 volts to a high voltage of 1.6volts. On the other hand, the processor when operating in deep sleepmode has no problem maintaining that state necessary to resume computingeven though functioning at a core voltage of 0.925 volts, the lowestvoltage which the regulator can provide. Thus, although the voltageregulator 11 may typically provide a range of varying output voltagelevels, the lowest voltage at which a processor is specified forconducting computing operations is typically significantly above thelowest value which the regulator is capable of furnishing.

In order to reduce power usage in one embodiment of the presentinvention, in response to a control signal indicating that the processoris about to go into the deep sleep state, the value at the input 15 isfurnished by the circuit 13 to the regulator causing the regulator 11 togenerate its lowest possible output voltage level for the deep sleepcondition. In one exemplary embodiment, the high and low voltagesgenerated in a computing mode are 1.6 volts and 1.2 volts while the deepsleep voltage is 0.925 volts.

Although the voltage level furnished by the regulator 11 for the deepsleep mode of the processor might appear to be only slightly lower thanthat furnished in the lowest operating condition for the exemplaryprocessor, the reduction in power usage is quite significant. Becauseboth the voltage and the leakage current are reduced, the reduction inpower is approximately equal to the ratio in voltage levels raised tothe power of about three to four. Over any period of processor useinvolving the deep sleep state, such a reduction is quite large.

One problem with this approach to reducing power is that it does notreduce the voltage level as far as might be possible and, thus, does notconserve as much power as could be saved. This approach only reduces thevoltage level to the lowest level furnished by the regulator. Thisvoltage is significantly greater than appears to be necessary for aprocessor which also dynamically regulates voltage furnished duringcomputing operations to save power. Two criteria control the level towhich the core voltage may be reduced in deep sleep. The level must besufficient to maintain state that the processor requires to functionafter returning from the deep sleep state. The level must be one thatcan be reached during the times allowed for transition to and from thedeep sleep mode.

The first criterion is met so long as values of state stored are notlost during the deep sleep mode. Tests have shown that a core voltagesignificantly below one-half volt allows the retention of the memorystate of a processor. Thus, using this criterion, it would be desirableto reduce the core voltage to a value such as one-half volt or lower.

However, depending on system configuration, the time allowed totransition to and from deep sleep in an X86 processor can be as low as50 microseconds. Depending on the capacitive load of the particularcircuitry, a voltage variation of about 0.5 to 0.6 volts may take placeduring this time in one exemplary configuration.

Thus, if the exemplary processor is operating at its lowest processingcore voltage of 1.2 volts, its core voltage may be lowered in the timeavailable to 0.6-0.7 volts. On the other hand, if the processor isoperating at a processing core voltage of 1.5 volts, its core voltagemay only be lowered in the time available to 0.9-1 volts. Consequently,it is desirable that the core voltage furnished during deep sleep belowered to a level which may be below the level provided by a typicalvoltage regulator but which varies depending on the core operatingvoltage from which it transitions.

This desirable result may be reached utilizing a circuit such as thatdescribed in FIG. 4. The circuit of FIG. 4 includes a feedback network41 for controlling the level of voltage at the output of the regulator11. Prior art regulators such as the Maxim 1711 provide a feedbackterminal and describe how that terminal may be utilized with aresistor-voltage-divider network joined between the output terminal andground to raise the output voltage level.

The embodiment of the present invention illustrated in FIG. 4 utilizesthe same feedback terminal and a similar resistor-voltage-dividernetwork but joins the divider between the output terminal and a sourceof voltage 42 higher than the normal output voltage of the regulator toforce the output voltage level to a lower value rather than a higherlevel. The particular source voltage and the particular resistor valuesmay be selected to cause the voltage level at the output of theregulator to drop from a particular output value to a desired value suchas 0.6 volts when transitioning from a computing level of 1.2 volts.

By appropriate choice of the resistor values of the divider network 41and the source 42, the voltage drop provided by such a divider networkaccomplishes the desired result of providing an output voltage for thedeep sleep mode of operation that varies from the previous processorcomputing core voltage by an amount attainable during the transitionperiod available. In one embodiment, resistor 43 was chosen to be 1Kohms, resistor 45 to be 2.7 Kohms, and source 42 to be 3.3 volts. Suchvalues cause the voltage drop into deep sleep mode to be between 0.5 and0.6 volts whether beginning at core voltages of 1.2 or 1.6 volts. On theother hand, by using a higher value of voltage at source 45 andadjusting the values of resistors 41 and 43, the increments of voltagedrop reached from different starting voltages to final deep sleepvoltage values at the terminal 12 may be brought closer to one another.

It should be noted that the circuitry of FIGS. 3 and 4 may be combinedso that both input selection and output adjustment are both used toadjust the core voltage value produced by a voltage regulator for deepsleep mode in particular instances where the load capacitance isrelatively low.

Prior art voltage regulators function in at least two different modes ofoperation. A first mode of operation is often referred to as “low noise”or “continuous” mode. In this mode, the regulator responds as rapidly aspossible to each change in voltage thereby maintaining the outputvoltage at the desired output level as accurately as possible. In orderto maintain this mode of rapid response, regulators consume a certainamount of power. When a regulator is supplying a significant amount ofpower to the load, the power required to operate in continuous mode isrelatively small. But, when a regulator is supplying a small amount ofpower to the load, the power used to operate the regulator in continuousmode becomes significant, and reduces the efficiency of the regulatorsignificantly. It is common for regulators operating in the continuousmode to transfer charge from the supply capacitors back into the powersource when the output voltage is changed from a higher voltage to alower voltage. The regulator can later transfer that charge back to theregulator output capacitors. Thus, most of the charge is not wasted.

A second mode of operation by voltage regulators is often referred to as“high efficiency,” “burst,” or “skip” mode. In this mode, a regulatordetects the reduction in load requirements (such as that caused by atransition into the deep sleep state) and switches to a mode whereby theregulator corrects the output voltage less frequently. When there is anincrease in load requirements, the regulator switches back to thecontinuous mode of regulation during which more rapid correction occurs.This has the positive effect of reducing the power consumed by theregulator during deep sleep thereby increasing the regulator efficiencyand saving system power. But, as a result of reducing the regulatorresponse rate, there is more noise on the regulator output.

It is common for regulators operating in the high efficiency mode todrain the charge on the supply capacitors during a high to low voltagetransition on the power supply output or to allow the load to drain thecharge. Thus, the charge is wasted during high to low voltagetransitions.

It is typical to operate a voltage regulator in the high efficiencymode. Consequently, there is some waste of power whenever a regulatedprocessor goes into the lower voltage deep sleep mode. If the processoris constantly being placed in deep sleep mode, then the loss of powermay be quite high. Different operating systems may increase the waste ofpower by their operations. For example, an operating system that detectschanges in operation through a polling process must constantly bring aprocessor out of deep sleep to determine whether a change in operatingmode should be implemented. For many such systems, such a system causesan inordinate amount of power waste if a processor would otherwise spendlong periods in the deep sleep mode. On the other hand, an operatingsystem that remains in deep sleep until an externally-generatedinterrupt brings it out of that state wastes power through operating theregulator in the high efficiency mode only when the processor is placedin the deep sleep state.

The present invention utilizes the ability of regulators to function inboth the high efficiency mode and the continuous mode to substantiallyreduce power wasted by transitioning between a computing and a lowervoltage deep sleep mode. Although regulators have not been dynamicallyswitched between high efficiency and continuous modes, in one embodimentof the invention, an additional controlling input 50 as shown in FIG. 5is added to the regulator for selecting the mode of operation of theregulator based on whether the processor being regulated istransitioning between states. If the regulator receives a control signal51 indicating that the processor is to be placed into the deep sleepmode, for example, then a regulator operating in the high efficiencymode immediately switches to the continuous mode during the voltagetransition. Assuming that the regulator returns the charge to thebattery during continuous mode, this has the effect of reducing thewaste of power caused during the transition. Once the transition hascompleted, the regulator switches back to the high efficiency state foroperation during the deep sleep mode of the processor.

For regulators that do not conserve capacitive charge by transferringthe charge to the battery, a circuit for accomplishing this may beimplemented or a capacitor storage arrangement such as a charge pump 53for storage may be added. Alternatively, when transitioning to deepsleep, the regulator could switch to a mode where the regulator does notactively drive the voltage low but allows the capacitor charge to drainthrough the load. The selection of power savings modes is dependent onthe processor leakage current, the voltage drop between the operatingand deep sleep voltages, and the efficiency of the regulator intransferring charge from the capacitors to the power source and thenback. If the leakage current is not sufficient to bring the voltage downmore than (1-efficiency)*(deep sleep voltage drop) during the deep sleepinterval, then it is more advantageous to use the load to drain thecharge on the capacitors. Otherwise, the charge on the capacitors shouldbe transferred back to the power source.

The control signal utilized may be the same control signal (stop clocks)that signals the transition into the deep sleep state if the method isto be used only for transitions between operating and deep sleep states.Alternatively, a control signal generated by a particular increment ofdesired change may be utilized for voltage changes within the computingrange of the processor as well as the transition to deep sleep mode.

Although the present invention has been described in terms of apreferred embodiment, it will be appreciated that various modificationsand alterations might be made by those skilled in the art withoutdeparting from the spirit and scope of the invention. The inventionshould therefore be measured in terms of the claims which follow.

1. A computer system comprising: a processing unit; circuitry coupled tothe processing unit, said circuitry configured to provide to saidprocessing unit: a sleep voltage; a first operating voltage; and asecond operating voltage that is less than the first operating voltage;wherein said computer system has a first transition time fortransitioning from said sleep voltage to said first operating voltage;wherein said computer system has a second transition time fortransitioning from said sleep voltage to said second operating voltage;wherein said second transition time is within an allowed time fortransitioning from a sleep state to an operating state; and wherein saidfirst transition time is greater than said allowed time.
 2. A computersystem as recited in claim 1 wherein said allowed time is based on aconfiguration of said computer system.
 3. A computer system as recitedin claim 1 wherein said allowed time is based on timing requirements ofsaid computer system.
 4. A computer system as recited in claim 3,wherein said timing requirements are based on interrupt response times.5. A computer system as recited in claim 1 wherein said first and secondtransition times are based on respective first and second voltage ramptimes.
 6. A computer system as recited in claim 1 wherein said sleepvoltage is sufficient to maintain state of said processing unit but isnot sufficient to maintain processing activity in said processing unit.7. A method of operating a computer processor, said method comprising:transitioning from providing a sleep voltage to said computer processorto providing a first operating voltage to said computer processor withinan allowed time for transitioning from a sleep state to an operatingstate; and transitioning from said providing said first operatingvoltage to said computer processor to providing a second operatingvoltage to said computer processor, wherein a transition time forchanging from said sleep voltage directly to said second operatingvoltage is greater than said allowed time for transitioning from saidsleep state to said operating state.
 8. A method in accordance withclaim 7 wherein said second operating voltage is greater than said firstoperating voltage.
 9. A method in accordance with claim 7 furthercomprising: enabling a system clock to said computer processor whenproviding said first operating voltage to said computer processor; anddisabling said system clock to said computer processor when providingsaid sleep voltage to said computer processor.
 10. A method inaccordance with claim 7, wherein said sleep voltage is sufficient tomaintain state of said computer processor but is not sufficient tomaintain processing activity in said computer processor.
 11. A computersystem comprising: a processor; an adjustable voltage supply configuredto output to said processor: a first and a second sleep voltage; a firstoperating voltage responsive to a transition from said first sleepvoltage: and a second operating voltage responsive to a transition fromsaid second sleep voltage, wherein said adjustable voltage supply isconfigured to generate a voltage transition from said second sleepvoltage to said first operating voltage in a time period greater than atime period allowed for transition from a sleep state to an operatingstate of said computer system.
 12. A computer system as recited in claim11, wherein said adjustable voltage supply is further configured tooutput to said processor a second operating voltage that, based on arate of transitioning from said sleep voltage to said second operatingvoltage, is achievable from said sleep voltage within said allowed timefor transitioning from said sleep state to said operating state.
 13. Acomputer system as recited in claim 11 wherein said allowed time isbased on a configuration of said computer system.
 14. A computer systemas recited in claim 11 wherein said adjustable voltage supply comprisesa voltage regulator.
 15. A computer system comprising: a processingunit; circuitry coupled to the processing unit, said circuitryconfigured to provide to said processing unit: a first sleep voltage anda second sleep voltage; a first operating voltage when transitioningfrom said first sleep voltage; a second operating voltage whentransitioning from said second sleep voltage, wherein said firstoperating voltage is greater than said second operating voltage andwherein said first sleep voltage is greater than said second sleepvoltage: and wherein a voltage transition from said second sleep voltageto said first operating voltage is greater than a time allowed fortransition from a sleep state to an operating state of said computersystem.
 16. A computer system as recited in claim 15, wherein saidcircuitry is further configured to provide to said processing unit: saidfirst sleep voltage when transitioning from said first operatingvoltage; and said second sleep voltage when transitioning from saidsecond operating voltage.
 17. A computer system as recited in claim 15wherein a voltage difference between said first operating voltage andsaid first sleep voltage is approximately equal to a voltage differencebetween said second operating voltage and said second sleep voltage. 18.A computer system comprising: means for processing; means for supplyinga voltage coupled to said processing means for providing to saidprocessing means: a first and a second sleep voltage; a first operatingvoltage responsive to a transition from said first sleep voltage; and asecond operating voltage responsive to a transition from said secondsleep voltage, wherein said means for supplying a voltage comprisesmeans to generate a voltage transition from said second sleep voltage tosaid first operating voltage in a time period greater than a time periodallowed for transition from a sleep state to an operating state of saidcomputer system.
 19. The computer system of claim 18 wherein: said meansfor supplying a voltage are further for providing to said means forprocessing: said first sleep voltage re responsive to a transition fromsaid first operating voltage; and said second sleep voltage responsiveto a transition from said second operating voltage.
 20. The computersystem of claim 18 wherein a voltage difference between said firstoperating voltage and said first sleep voltage is approximately equal toa voltage difference between said second operating voltage and saidsecond sleep voltage.
 21. The computer system of claim 18 wherein saidfirst operating voltage is greater than said second operating voltageand wherein said first sleep voltage is greater than said second sleepvoltage.